Focused ultrasound (i.e., acoustic waves having a frequency greater than about 20 kilohertz) can be used to image internal body tissues or, at high intensity, to generate thermal ablation energy to treat tissue such as tumors. By way of illustration, FIG. 1 is a simplified schematic representation of an exemplary focused ultrasound system 100 used to generate and deliver a focused acoustic energy beam 102 to a targeted tissue mass 104 in a patient 106. The system 100 employs an ultrasound transducer 108 that is geometrically shaped and physically positioned relative to the patient 106 in order to focus the ultrasonic energy beam 102 at a three-dimensional focal zone located within the targeted tissue mass 104. The transducer 108 may be substantially rigid, semi-rigid, or substantially flexible, and can be made from a variety of materials, such as plastics, polymers, metals, and alloys. The transducer 108 can be manufactured as a single unit, or, alternatively, be assembled from a plurality of components. While the illustrated transducer 108 has a “spherical cap” shape, a variety of other geometric shapes and configurations may be employed to deliver a focused acoustic beam, including other non-planar as well as planar (or linear) configurations. The dimensions of the transducer may vary, depending on the application, between millimeters and tens of centimeters.
The transducer 108 may include a large number of transducer elements 110, arranged in a one- or two-dimensional array or other regular manner, or in an uncoordinated fashion. These elements 110 convert electronic drive signals into mechanical motion and, as a result, into acoustic waves. They may be made, for example, of piezoelectric ceramics or piezo-composite materials, and may be mounted in silicone rubber or another material suitable for damping the mechanical coupling between the elements 110. The transducer elements 110 are connected via electronic drive signal channels 112 to a control module 114, which drives the individual transducer elements 110 so that they collectively produce a focused ultrasonic beam. More specifically, the control module 114 may include a beamformer 116 that sets the relative amplitudes and phases of the drive signals in channels 112. In conventional focused ultrasound systems containing n transducer elements, the beamformer 116 typically contains n amplifiers 118 and n phase control circuits 120, each pair driving one of the transducer elements 110. The beamformer 116 receives a radio frequency (RF) input signal, typically in the range from 0.1 MHz to 3 MHz, from frequency generator 122. The input signal may be split into n channels for the n amplifiers and phase circuits 118, 120 of the beamformer 116. Thus, in typical conventional systems, the radio frequency generator 122 and the beamformer 116 are configured to drive the individual elements 110 of the transducer 108 at the same frequency, but at different phases and different amplitudes, such that the transducer elements 110 collectively form a “phased array.”
The acoustic waves transmitted from the transducer elements 110 form the acoustic energy beam 102. Typically, the transducer elements are driven so that the waves converge at a focal zone in the targeted tissue mass 104. Within the focal zone, the wave energy of the beam 102 is (at least partially) absorbed by the tissue, thereby generating heat and raising the temperature of the tissue to a point where the cells are denatured and/or ablated. The location, shape, and intensity of the focal zone of the acoustic beam 102 is determined, at least in part, by the physical arrangement of the transducer elements 110, the physical positioning of the transducer 108 relative to the patient 106, the structure and acoustic material properties of the tissues along the beam path between the transducer 108 and the target tissue 104, and the relative phases and/or amplitudes of the drive signals. Setting the drive signals so as to focus the acoustic energy at a desired location is a process known as “electronic steering” of the beam 102. The amplification or attenuation factors α and the phase shifts φ imposed by the beamformer 116 and used to steer the beam are computed in a controller 124, which may provide the computational functions through software, hardware, firmware, hardwiring, or any combination thereof. For example, the controller 124 may utilize a special-purpose digital signal processor or a general-purpose computer programmed with software in a conventional manner. In certain embodiments, the computation is based on image data obtained with a magnetic resonance imaging (MRI) apparatus or other imager (not shown). Further, the controller 124 may be in communication with a user interface 126 that facilitates the selection of the focus location or other treatment parameters.
Phased-array transducers provide the greatest beam-steering capability when each transducer element 110 can be controlled independently through a separate drive signal channel 112, as illustrated in FIG. 1. This flexibility comes, however, at a price, as large numbers of electronic channels are costly. Thus, as the number of elements increases, the ability to drive them independently becomes concomitantly less practical for complexity and cost reasons. To curtail cost increases resulting from larger numbers of transducer elements, many applications exploit system symmetries to drive multiple elements with a single channel. An axis of symmetry may be defined, for example, by the direction of the acoustic beam propagation when all elements of the array are driven in phase. The transducer elements may be connected to (a smaller number of) drive signal channels in a way that reflects, for example, a corresponding radial symmetry around the intersection of this axis with the transducer surface. Such symmetric transducers achieve a reasonable trade-off between high steering capability along one axis (e.g., steering of the focal length at the cost of very limited lateral steering, or vice versa) and low system complexity in many applications. However, they are unduly limiting when the transducer cannot be freely moved, e.g., as a consequence of certain anatomical barriers. Accordingly, there is a need for accommodating large numbers of small transducer elements in any array without the burden of separately driving them, but also without sacrificing the beam focusing and steering capabilities needed in specific clinical applications.